This excerpt taken from the BP 6-K filed Mar 13, 2006.
Assets carried at amortized cost If there is objective evidence that an impairment loss on loans and receivables carried at amortized cost has been incurred, the amount of the loss is measured as the difference between the assets carrying amount and the present value of estimated future cash flows discounted at the financial assets original effective interest rate. The carrying amount of the asset is reduced, with the amount of the loss recognized in administration costs.
If, in a subsequent period, the amount of the impairment loss decreases and the decrease can be related objectively to an event occurring after the impairment was recognized, the previously recognized impairment loss is reversed. Any subsequent reversal of an impairment loss is recognized in the income statement, to the extent that the carrying value of the asset does not exceed its amortized cost at the reversal date.